Heretofore, polyimides have been widely used as protecting materials or insulating materials in the electric and electronic fields, by virtue of high mechanical strength, heat resistance and solvent resistance, as their characteristics. However, developments in the electric and electronic fields in recent years have been remarkable, and increasingly high properties have been required also for materials to be used. Especially in the application to alignment films for liquid crystal display devices, polyimides have been mainly employed because of the uniformity and durability of the coating film surface. However, in an attempt for high densification and high performance of liquid crystal displays, the surface properties of polyimide coating films have become important, and it has become necessary to impart a new property which conventional polyimides do not have.
A liquid crystal display device is a display device utilizing an electro-optical change of liquid crystal and has undergone a remarkable development as a display device for various displays in recent years in view of the characteristics such that it is small in size and light in weight as a device and its power consumption is small. Especially, a twisted nematic type (TN-type) electric field effect liquid crystal display device is a typical example, wherein nematic liquid crystal having positive dielectric anisotropy is employed so that liquid crystal molecules are aligned in parallel with a substrate at the interface of each of a pair of mutually opposing electrode substrates, and the two substrates are combined so that the directions for alignment of liquid crystal molecules are orthogonal to each other.
In such a TN-type liquid crystal display device, it is important that long axis directions of liquid crystal molecules are aligned uniformly in parallel with the substrate surface and further that the liquid crystal molecules are aligned with a certain inclined alignment angle (hereinafter referred to as a tilt angle) against the substrate. As typical methods for aligning liquid crystal molecules in such a manner, two methods have been known heretofore.
The first method is a method which comprises vapor depositing an inorganic substance such as silicon oxide from an oblique direction to a substrate to form an inorganic film on the substrate, so that the liquid crystal molecules are aligned in the direction of vapor deposition. This method is not industrially efficient, although uniform alignment with a constant tilt angle can be obtained.
The second method is a method which comprises forming an organic coating film on a substrate surface, and rubbing its surface in a predetermined direction with a cloth of e.g. cotton, nylon or polyester, so that liquid crystal molecules are aligned in the rubbing direction. By this method, constant alignment can be obtained relatively easily, and industrially, this method is primarily employed. As the organic film, polyvinyl alcohol, polyoxyethylene, polyamide or polyimide may, for example, be mentioned. However, from the viewpoint of the chemical stability, thermal stability, etc., polyimide is most commonly employed.
In the field of liquid crystal alignment films, it has been difficult to obtain a high tilt angle constantly by the method of rubbing an organic film such as polyimide. As a means to solve such difficulty, JP-A-62-297819 proposes a treating agent for liquid crystal alignment made of a mixture of a long chain alkyl compound with a polyimide precursor. Further, JP-A-64-25126 discloses a treating agent for liquid crystal alignment made of a polyimide using, as a starting material, a diamine having an alkyl group. Thus, many attempts have been made to increase the tilt angle of liquid crystal by introducing an alkyl group into polyimide, and it has been made possible to increase the tilt angle.
However, with the above mentioned liquid crystal alignment film having an alkyl group introduced into polyimide, the thermal stability of the tilt angle was inadequate. Namely, conventional polyimide alignment films having an alkyl group introduced, had a problem that the tilt angle tended to decrease when heated at a temperature of at least the isotropic temperature of liquid crystal (hereinafter referred to as isotropic treatment), although the tilt angle immediately after injection of liquid crystal can be increased. Especially when the tilt angle is high, or the curing temperature during formation of the alignment film is low, the decrease in the tilt angle by the isotropic treatment becomes remarkable. Further, when a polyimide film is formed on a substrate, it is common to have the film baked at a high temperature of from 200 to 300.degree. C., and there was a problem that since the heat resistance of the alkyl side chain itself was not sufficient, the tilt angle tended to decrease or non-uniformity tended to result especially in high temperature baking. These problems are extremely important questions to be solved to accomplish a uniform liquid crystal display with a higher contrast in the future liquid crystal display device, and a polyimide alignment film has been very much desired which not only has a high tilt angle but also presents a thermally more stable tilt angle.